The use of polymeric particles and magnetic particles to bind a compound has long been known and used in industrial and laboratory procedures. For example, the Merrifield resins, crosslinked styrene-divinylbenzene spheroidal beads, were among the earliest and most widely used modern substrate particles. They were used in organic synthesis, for heterogenizing homogeneous catalysts and in biochemical reactions. Since the Merrifield resins were fairly large, they could easily be separated by filtration. In some fields, however, it is desirable to use colloidal sized particles because the material to be bound is scarce, expensive or is to be used in a procedure where larger particles are not desirable. This is particularly true in the biochemical field. When particles are of colloidal size, however, their separation from liquid medium by filtration can become lengthy and difficult. In particular, colloidal particles tend to coat the surface of the filter and slow the process. The use of magnetic particles, specifically magnetic particles having a polymeric coating, has found great utility because such particles can be magnetically gathered to one side of a reaction vessel and the bulk of the reaction medium simply decanted. (The word "particles" as used herein encompasses spheres, spheroids, beads and other shapes as well. These words are used interchangeably unless otherwise specified.) The use of coated magnetic particles has found a particular utility in biological applications, especially where antibodies are bound to the surface coating of the particles. The bound antibodies may then be used to capture a specific biological substance from a test sample containing numerous biological samples or to capture undesired species from the test sample, leaving the desired species in the sample.
The categories of coated magnetic particles, also known as magnetic spheres or beads, can be divided into four general classes.
1. Core-and-shell beads with a magnetic core and a hard shell coating of polymerized monomer or a silanizing agent. See Rembaum U.S. Pat. No. 4,267,234 (polyglutaraldehyde shell around ferrofluid core particles); Czerlinski U.S. Pat. No. 4,454,234 (suspension or emulsion polymerized coating around submicron magnetic particles); Whitehead et al. U.S. Pat. No. 4,554,088 (silanized magnetic oxide particles of polydisperse size and shape); and Margel et al. U.S. Pat. No. 4,783,336 (suspension polymerized polyacrolein around ferrofluid particles).
2. Core-and-shell beads with a magnetic core and a loose shell of random coil or globular polymer which may or may not be crosslinked. See Molday U.S. Pat. No. 4,452,773 (dextran coating around ferrofluid particles) and Owen et al. U.S. Pat. No. 4,795,698 (protein such as bovine serum albumin around ferrofluid particles.
3. Magnetic latex materials formed by uniformly embedding ferrofluid particles in polystyrene latex particles. See Daniel et al. U.S. Pat. No. 4,358,388.
4. Porous polymer particles filled with magnetic materials such as polymer-ferrite or polymer maghemite composite systems. See K. Nustad et al. "Monodisperse Polymer Particles In Immunoassays And Cell Separation", Microspheres: Medical and Biological Applications, A. Rembaum and Z. Tokes, eds. (Boca Raton, Fla.: CRC Press, 1988) pages 53-75; C. D. Platsoucas et al., "The Use Of Magnetic Monosized Polymer Particles For The Removal Of T Cells From Human Bone Marrow Cell Suspensions", ibid. at pages 89-99; and International Patent Publication No. WO 83/03920, Ughelstad et al. (polymer coated magnetic particles prepared by treating compact or porous particles with a solution of iron salts and the use of such particles for medical, diagnostic or other purposes).
The usefulness of most polymer coated magnetic beads in medical and biological applications has been limited by practical considerations such as the uniformity of particle size and shape, the need for the biological reagent to be strongly bound to the particle, a preference for hydrophilic polymer coatings as opposed to hydrophobic coatings, and whether or not the coating is biodegradable. While biodegradability is of particular importance where a biological reagent is to administered in vivo, it is also important in various cell sorting, separation and assay procedures. The most desirable coated magnetic particles would have the following features.
1. The particles should be as small as possible in order to maximize the surface area on which the biological reagent is coated, but the particles should still be easily separable with a small magnet. Small size and large surface area are desirable in order to use the least possible quantity of particles to remove the targeted substance; e.g., to interact with on the order of 10.sup.6 cells per sample in one step, thereby avoiding sequential additions and work-ups.
2. There should be a low non-specific binding of the antibody-coated particles to cell surfaces. The particle surface should be hydrophilic or covered with a coating of a hydrophilic substance to which the antibody is attached.
3. The polymer and antibody layers on the particles should be covalently bound to each other in order to reduce dissociation and conformational changes.
4. The coating on the magnetic particles and any molecular chains which link an antibody to the polymer surface should be metabolizable.
5. In positive selection of cells, a mechanism for quickly and easily recovering viable cells from the magnetic particles should be available in order that recovered cells can be cultured.
6. In the negative selection of cells, the antibody-coated particles should be sterile so that the remaining cells can be cultured.
In addition to magnetic particles, there is also a need for polystyrene latex (PSL) particles which have been coated with hydrophilic polymer coatings to which antibodies can be subsequently bound. These polymer coated PSL particles can be used in bead-based cell population analyses and immunoassays. However, non-magnetic PSL particles, as made, usually have a relatively low density of various functional groups such as carboxyl or amino groups. Consequently, covalent coupling of coating materials such as dextran or gelatin to the surface of PSL particles is not satisfactory.
The various particles described above have been used in the biological arts to immobilize a variety of biological substances, particularly antibodies. In using such particles, immobilization of antibodies by covalent coupling is preferred to immobilization by antibody adsorption which requires careful and separate adjustment of pH and antibody concentration for each monoclonal antibody used. P. Bagchi et al., J. Colloid Interface Sci., 83:460-478 (1981); J. Lyklema, Colloids and Surfaces, 10:33-42 (1984); M. D. Bale et al., J. Colloid Interface Sci., 125:516-525 (1988); C. C. Ho et al., ibid., 121:564-570 (1988); "Proteins at Interfaces: Physicochemical and Biochemical Studies", ACS Symposium Series, No. 343, J. L. Brash and T. A. Horbett, Eds. (Washington: Amer. Chem. Soc., 1987); W. Norde, Adv. Coll. Interface Sci., 25:267-340 (1986); A. V. Elgersma et al., Abstracts of the 198th Amer. Chem. Soc. Meeting, Miami Beach, Fla., Sept. 10-15, 1989, COLL 0131; and D. E. Brooks, Annenberg Center for Health Sciences and H. B. Wallis Research Facility at Eisenhower Latex Conference, Orlando, Fla., Dec. 4-5, 1989. However, even when the pH and antibody are carefully controlled, there is little assurance that the orientation of adsorbed antibody will be such that an active adsorbed antibody will result. Adsorbed antibodies also have long term storage problems arising from antibody desorption from the particles' surfaces. Furthermore, proteins, such as antibodies, tend to achieve maximum adsorption on hydrophobic surfaces at or near the pI of the protein. However, if electrostatic interactions between charge groups are important, then the adsorbing surface and the adsorbate should have net opposite charges. Covalent coupling methods, on the other hand, are not as sensitive to these conditions.
Covalent coupling methods have been used with particles of magnetite embedded in carboxy-modified latex subsequently coated with aminodextran and derivitized with a number of antibodies. R. S. Molday et al. FEBS. Lett., 170:232-238 (1984). If the antibody is of IgG isotype, the covalent coupling method assures that the linkage between the antibody and the particles occurs at the antibody's Fc or hinge region, and not at the antibody's Fab region. If the antibody is of pentameric IgM isotype which has only Fab regions exposed, the coupling of one Fab region to the particle will still leave four Fab regions exposed and available for reaction.
This invention provides for the preparation of magnetic and non-magnetic particles having a biodegradable coating to which can be attached pendent biological substances, such as monoclonal antibodies. The particles of the invention can be used in various cell separation and assay methodologies. Biodegradability in the coating used on the magnetic or latex core material is important in cell separation technology. For example, antibodies may be conjugated to gelatin coated magnetic particles such as manganese ferrite particles. These particles would thus contain a proteinaceous coating and a manganese-iron oxide core, all of which are biodegradable. In a positive cell selection procedure using such particles, once the desired cell has been isolated from other cells, the particles and coating can be allowed to degrade in a manner such that the cells are kept viable and can be cultured for further use. Alternatively, the enzyme collagenase can be used first to release the core material (magnetic or latex) by digestion of the gelatin coating. The core material can then be removed from the cell suspension before culturing the cells. In the negative selection of cells with such biodegradable beads, the beads can be left in the cell suspension from which targeted cells were removed without compromising the viability of the remaining cells. For example, in bone marrow purging operations using biodegradable magnetic beads, there is less concern about leaving behind some beads in the purged marrow that is to be transplanted in a patient. Currently, synthetic polymer-magnetite particles prepared by Ughelstad et al, International Patent Publication No. WO 83/03920, and conjugated with antibody are being used in bone marrow purging. The polymer is not biodegradable and imparts a hydrophobic surface to these beads. This hydrophobicity, which is not present in the gelatin coated particles of the claimed invention, is responsible for non-specific interactions between the beads and cells. As a result of this non-specific interaction, the selectivity is poor and more beads must be used to attain the desired level of treatment. The claimed invention avoids these problems.